Green Arrow: I couldn't beat you by myself. So I'm teaming up with Deathstroke.
Prometheus: But didn't he murder your mom? I didn't do anything nearly that bad to you.
Deathstroke: We worked through it mate. We're best friends again.
Seriously though...could Prometheus (or Vigilante) be Deathstroke's son Joe (Jericho?). Legends of Tomorrow introduced Slade's future son Grant but I believe Joe was the one Slade mentioned in the first season of Arrow that supposedly died.
Making Prometheus or Vigilante Joe Wilson is about the only way to salvage one of these 2 and make them interesting.
He should have been the one to stop Ra Ghoul though...